1. Field of the Invention
The present invention relates to a creep preventing device for vehicles equipped with an automatic transmission.
2. Description of the Prior Art
With a shift lever being set to the drive position (forward-movement position) during a stop, vehicles equipped with an automatic transmission develop a so-called creep phenomenon which pulls the vehicle forward against the intention of the driver due to the dragging torque of a torque converter. Such a creep force applies a brake force to the engine during the idle operation. To hold the number of revolutions of the engine during the idle operation at a reference value, therefore, it is required to increase the opening degree of a throttle valve during the idle operation to a certain extent to compensate the brake force. This increases the fuel consumption as a matter of course and was one of primary causes to deteriorate the fuel efficiency of vehicles equipped with an automatic transmission.
The assignee of the present invention therefore has previously proposed an improvement in that the idle operating condition of the vehicle is detected and the pressure exerting on a starting clutch is by-passed to an oil tank to interrupt power transmission between the engine and drive wheels, so that the load upon the engine assumes the same level as that in vehicles equipped with a manual transmission. Such an improvement has, however, accompanied a drawback as described below. That is, if response characteristics are taken into consideration at the time of starting, the pressure exerting on the starting clutch should not be reduced to zero, but should rather be maintained at a weak pressure Po which is not so great as the force of a return spring in the clutch, during the condition in which the creep is to be prevented from developing. This pressure Po is very weak, i.e., about 1 atm. To maintain the pressure Po as precisely as possible, a relief valve is suitably used as a means for escaping the pressure into the oil tank. Namely, the construction is such that the pressure is introduced into a spring chamber via a throttle, and a circuit for communicating the spring chamber with the oil tank is opened or closed by a pilot valve, in order that the valve is urged toward the opening side by the pressure and that the valve is urged toward the closing side by the force of spring. However, since the pressure Po is very weak as mentioned above, the spring which urges the valve toward the closing side must also have a very weak force. Therefore, if the oil contains impurities such as dust and dirt, the valve fails to operate smoothly, the creep preventing mechanism fails to operate, and the starting clutch is reengaged with decreased reliability. This results from the weak spring force as described above; i.e., the valve fails to return to the rest position, or its returns to the rest position being delayed. This problem can be solved if a spring having an increased force is employed, making, however, it necessary to use the valve of a large size to prevent the creep.
In view of the above-mentioned problems, the present invention provides a creep preventing device for vehicles equipped with an automatic transmission, in which the creep preventing valve operates with increased reliability, since the creep preventing valve, which permits the pressure for engaging the starting friction elements to escape into the oil tank during the idle operation, is returned to the inoperative position not only by the force of a return spring but also by a hydraulic servo force.
According to a first feature of the present invention, to achieve the above-mentioned object, the creep preventing valve is so constructed as to move in two directions, i.e., opening direction and closing direction, by the pressure of the source of hydraulic pressure. This makes it possible to utilize the hydraulic servo force for returning the creep preventing valve to the inoperative position, eliminating such probabilities that the valve is not returned to the rest position or is returned with some delay due to weak spring force. Accordingly, the creep preventing valve need not be constructed in a large size to reinforce the spring force, and increased reliability in operation can be obtained.
Further, the below-mentioned inconvenience develops in the automatic transmission in which the pressure of the starting clutch is by-passed to the oil tank during the idle operation and a transmission gear train (such as low-speed gear train) at the time of idle operation is also used when the engine brake is applied. That is, there will be no problem when all of the systems are properly working. However, if the system is maintained in a condition intended for preventing the creep due to some trouble, the low-speed gear train is not put into use for applying the engine brake. Therefore, the vehicles which have a shift position at which the engine brake is applied with the low-speed gear train can be brought into halt relying upon the foot brake only. Such troubles may develop in cases where the sensor system becomes defective to erroneously judge the running condition as idle operation, where the actuator portions become defective to maintain the condition for preventing creep though the sensor systems and judging systems are properly functioning, and the like. In particular, the hydraulic pressure switching valve as a final actuator may lose smooth operation if dust and dirt are contained in the oil, and may not often return to the inoperative position since the return spring has a weak force.
This sort of trouble does not develop with the vehicles which do not have a shift position where the engine brake is applied with the low-speed gear train. With the vehicles of the type which applies the engine brake with a middle-speed gear train, however, the reduction ratio of the middle-speed gear train could not be arbitrarily reduced from the point of view in maintaining satisfactory braking performance, and this caused considerable restriction on improving its fuel consumption.
The present invention was accomplished in view of such problems, and its object is to provide a creep preventing device for vehicles equipped with an automatic transmission, which is capable of preventing the occurrence of creep phenomenon when the starting gear train is selected and which is capable of applying the engine brake with the starting gear train, wherein the function of engine brake is not lost even in case the creep preventing mechanism becomes defective at the operating position thereof.
According to a second feature of the present invention, for attaining the above-mentioned object, a creep preventing valve has an oil chamber which closes the creep preventing valve when a hydraulic pressure greater than a predetermined value is introduced thereinto, and to the oil chamber is connected an oil path which guides the hydraulic pressure from a signal generator means which produces a signal pressure in proportion to the vehicle speed.
Therefore, even if the electric signal system becomes defective or dust and dirt are contained in the oil and the valve is not closed during the idle operation, the valve can be reliably moved to the closing position by the hydraulic pressure, and the engine brake can be reliably applied.
The above-mentioned and other objects, features and advantages of the present invention will become obvious from the following description of preferred embodiments in conjunction with the accompanying drawings.